Re: Why does the limit use the early row lookup.

[Zhangzhigang]

Why does not the mysql developer team to do this optimization?

--- 12年4月20日，周五, Reindl Harald <h.reindl@stripped> 写道：

> 发件人: Reindl Harald <h.reindl@stripped>
> 主题: Re: Why does the limit use the early row lookup.
> 收件人: mysql@stripped
> 日期: 2012年4月20日,周五,下午3:50
> 
> 
> Am 20.04.2012 04:29, schrieb 张志刚:
> > My point is that the limit can use late row lookup:
> lookup rows after
> > checking indexes to optimize the select speed.
> > 
> > But the mysql optimizer do it with the early row
> lookup: lookup all rows
> > before checking indexes when the one fetch column is
> not in the indexes.
> > 
> > Tell me why?
> 
> because the mysql optimizer until now is really
> bad in many situations - order by rand() makes a
> temporary table wil ALL data as example even with limit
> 
> select * from table order by rand() limit 10;
> reads and writes the whole table to disk
> have fun with large tables :-)
> 
>